Insulated conductor



June 2, 1931.

D. R.l BRoBsT INSULATED CONDUCTOR Filed Nov. 10, 1925 Psfenied June 2,1931 UNITED STATES PATENT OFFICE DAVID) R. IBROBST, F NEW YORK, N. Y.,ASSIGNOR TO BELL TELEPHONE LAI-BORATORIES, INCORPORATED, OF NEW YORK, N.Y., A. CORPORATION 0F NEW YORK- msi-.mamma coNDfroron.

Application illed November 10, 1925. Serial No. 68,0578.l

This invention relates to electric conductors and particularly toiexible conductors and cables.

An object of the invention is to improve the insulation characteristicsof conductors insulated with cellulose acetate.

Another object of the invention is to improve the Skinningcharacteristicsof fibrous covered conductors impregnated with celluloseacetate.

Another object of the invention is to eliminate air-gaps and intersticesin theinsulation of a conductor.

Another object of the invention is to improve the quality and reduce thecost of lswitchboard cables.

Another object of the invention is to improve methds of impregnatingfibrous covered conductors with cellulose acetate.

S0 Heretofore, it ha's been proposed to enamel bare conductors withcellulose acetate lacquers as well as to treat fabrics with celluloseacetate in order to produce a glossy 'finish. It has also been suggestedto'impregn'ate fibrous covered conductors with cellulose acetate inorder to improve the'insulation characteristics of such conductors. Anexample of such a method is that disclosed in a copending application ofJohn H. White, Se-

' n ria1N0.750,718,med-November 19,1924.

Accordin to the method disclosed in Whites app 'caiton the fibrousinsulation upon a conductor is impregnated with a solution of celluloseacetate and dried preferably without allowing moisture to come incontact with the impregnated conductor during the drying operation.y Theresult of this process is to produce a conductor provided with a glossytube-like cover in which the '40, cotton fibres are enclosed in a solidmatrix of cellulose acetate. Such an insulating cover also it has beenfound that the cellulose acetate solution does not readily replace thelair in the lowerlayers of the fibrous material unless specialprecautions are taken. Various methods of treating the fibrous materialto overcome this difficulty have been developed but these have theobjection ofadding an extra ste to the process. f

A thoroug penetration of cellulose acetate is particularly desirable inthose cases where the impregnated conductor is to be subsequentlyskinned. When a thorough impregnation is not obtained the'fibrouscoveringl `it is applied.

In accordancel with one of its features, the

invention provides a new, and improved method of treating the individualconductors of a cable such as a telephone switchboard cable withcellulose acetate in which the im.- pregnation iscarried out so that thecellulose acetate permeates the complete structure being treated. Inthis manner, all the individual fibres of the covering receive fathorough coating of cellulose acetate, and all the elc- I ments of thecovering will adhere together. Such a structurehas been found tomaterially enhance the electrical insulating characteristics and theSkinning characteristics of the impregnated conductor. Briefly, this isaccomplished by impregnating the fibrous covered conductor with asolution ofy cellulose acetate, applying a potential between theconductor and the container holding the solution during theimpregnation, and drying.

The result of this process is to produce a conductor provided with aglossy tube-like cover in which the cotton fibres are enclosed in asolidmatrix of cellulose acetate, and in which air-gaps are.substantially eliminated.

The various features and advantages of the invention'are described indetail with ple.

reference tothe accompanying drawings in which: Fig. 1 is a lsectionalside view illustrating the method of treating an insulated conductor inaccordance with the invention.

Fig 2 is a front view in section of the ap aratus shown in Fig. 1.

s shown in the figures, a conductor 1 1nsulated with cotton or 4othersuitable `insu-A lating material is drawn from a supply reel 2 over asuitabley pulley or other guiding mea-ns 3 to lan impregnating tank 4containing a cellulose acetate bath 5. The impregnating tank 4 ispreferably composed of a suitable electricall conducting celluloseacetate-inert materia such as brass, for exams From the pulley 3 theconductor passes under the pulley 6, up and over the pulley 7 downandunder the pulley 8, u and out of the bath and tank and over t epulley 9. From the tank 4 the conductor passes to an impregnatiuf tank10, and drying chamber `11 to a suitable take-up reel 12.

The take-up reel 12 is preferably formed of an electrlcally conductingmaterial so .o that it may be employed as one ofthe elec-- trodes forapplying the potentialbetween the conductor and the bath containing theconductor during the impregnation treatment. When used in'this manner,the conductin core of the conductor being treated is attac ed tothemetallic core of the take-up reel 12.

A suitable binding post is attached to the impregnating chamber 4 vsuchas at 13.

The insulated conductor 1 may be preheated to remove moisture before theimpre nating process though this step is not consi eredessential tothe'invention. An number of layers of fibrous insulation may e employed.

For example', the conductormay be insulated y with a single layer ofcotton yarn of suicient through the impregnating tank 4, a potential.

-coarseness to secure tlae desired separation between the conductors ofthe cable to keep down the capacity therebetween. It is also preferableto'employ cotton yarn which is preferably not too highly hylgroscopicavoidmg the use of cotton which as been .bleached or otherwise treatedin such a wa as to increase its hygrosco ic properties. f desired,

either before or a er bein spun into yarn, the cotton may be tested anif found to contain an undesirable quantity of soluble hygrosco ic andsaline materials, it may be put throug a treat-ment of pure-hot water toremove such materials.

During the passage of the conductor is applied in accordancewith the'invention between suitable electrodes,`such as metallic reel 12 to whichthe conductin core of the conductor is attached andthe inding post 13,the reel 12, that is,'the conductor, being preferably made the anode orpositive electrode. With the application of such a. po-

erably have. such 'a composition, however,

that its viscosity will be such that with the rate of motion of theconductor, the temperature of the bath, and the potential employed athorough penetration will be pro'- duced. For general use, however, itmay be -said that the proportion of cellulose acetate to solvent in thebath'may be varied over Wide limits. Solutions prepared for example, bydissolvingabout 150 grams to 300 grams of dry ing a. specific gravity of0.797 at 60 F. have cellulose acetate pulp in a 'United States-gallon ofrefined acetone havbeen found suitable. This solution is preferablyallowed to settleV and the clear supernatant liquor drawn olf for use inthe impregnating-tank 4 while the gray undissolved sludge, if suchremains, is rejected.

The insulated conductor ispassed through the tank 4, containing thecellulose acetate v solution prepared in the manner described above, anynumber of times desired so as to become thoroughly penetrated with thesolution. In thel drawings the conductor.v is

shown as being given four passes thoroughy thebath in the tank 4.

' After passing through the 'tank 4 it has been. found that theconductor covering is thoroughly impregnated with cellulose acctate. i'In order to produce a satisfactory finish, however, it has been foundpreferable to pass the initially impregnated conductor -through aplurality of additional impregnation steps without the application of'apo-l tential. In order to carry out'this object,

the conductor after leaving the tank 4 is drawn under a suitable guidereel 14 through a cellulose acetate bath 15 contained in e tank 10. Thedistance between the outlet of tank'4 and the impregnating bath 15 andthe speed of the conductor 4arevfpreferably such as to prevent thedeposition of moisture, on the conductor before" it reaches the bath 15.The conductor then' asses Vthrou h the d ing chamber ll'whic may be, orexamp e, an electric furnace comprisingy a. refractory core 16 woundwith suitable resistance wire 17- and enclosed within a secondrefractory tube 18 over which suitable h the' `chamber 11 1s preferablyso regulate that heat'insulating material 19 is placed'. vThe passage ofthe conductor throu v chamber 11.

The subsequent impregnation treatments may be continued throughout annumber of steps tol produce the desired thickness of toproducesatisfactorycharacteristics. However, this set of conditionscannotbe said to after the initial ste-p insulation and the desiredsmooth and polished appearance of the conductor. Inv the figure, theconductor is shown asbeing given four passes through the impregnatingbathb employing the potential difference.

It has been found that when the insulated conductor is first passedthrough an impregnating process employing a potential difference betweenthe conductor and the container holding the bath the insulated conductoris thoroughly impregnated down to the wire and the iaws heretoforeencountered are obviated.

The particular operating conditions employed in any given instance willvary, of course, with the yworking conditions of the plant. A set ofconditions which has been found satisfactory for impregnating, wirehaving a covering-of two servings of cotton yarn, one being No. 50cotton yarn and the other being No. 30 two-ply yarn, has been found tobe the potentialldife-rence of 250 volts, and a rate of motion of aconductor of 10 feet er minute, the conductor being subject to te'action of the potential diierence while in contact with the celluloseacetate for approximately three minutes. The example given is one whichhas 'been found be preferred for all purposes over others but conditionsintermediate those given or differing widely Ifrom them and yet withinthe limits deiined by the claim may be used to secure the advantages ofthe invention, the conditions employed depending upon the particularconditions stance.

It will be obvious that the terminals for applying the potential'difference may be other than those represented in thegure of thedrawings. For example, the cathode might be an electrode in contact withthe hatrli instead of the container holding the The temperature of thecellulose acetate l bath which is employed, has not been found tosubstantially infiunce the results. obtained.

The use of an initial tem erature of the bath in the neighborhood olroom temperature has been found satisfactory. Asia. general rule, it hasbeen found that the tenriperatureA ofthe bath increases materially withtime as present in each iny, ance with the invention.. It has beenfound,

for example, that the AC mutual capacity Aand conductances between wiresin a pair are both lowered and theDC insulation resistance and ACbreakdown strength are materially increased. As an exainple 'of theincrease in insulating characterlstics which is produced by treating aninsulatingconductor in accordance with the invention, one insulatedconductor'was given 5 passes through cellulose acetate as show in thedrawings without applying'a potential difference and another conductorof the same type given the same treatment with the application ofpotential. The rate of motion ofthe conductor was approximately 17 feetper minute, applied potential difference between 230 and 250 volts andthe length of time during which the conductor was subjected to-thepotential d iiference approximately 11A, minutes. The fol-v lowing tablegives the results obtained: i Capacity in micro-micro farads ,in 50yfeet Tmp 85 85 85 85 Humidity 05% 75% 85% 90% 93% 75% 65% Conductortreated with applied potential 1,500 2,000 2,700 3,330 4,000 2,700 2,200Conductor treated without applied potential...T 1,900 2,300 3,050 3,7004,540 3,050 2,500 Percentage decrease-- 21% 1'3%11%% 10% 12% 11%%luo'omluctace in micro-mitos per 50.feet VN Conductor treated l withapplied potenv Jini. 2.2 2.2 2.3 '8.3 6.0 2.4 2.2

Conductor treated A without applied po. l tentil i 2.2 2.2 2.3 14.0 10.12.6 ".3 Percentage decrease... 0.0 A 0.0 d 0.0 40.61' 40.6 8.0 5.0

Insulation resistance in megohms per 50 feet Temp- Humidity vpotentreated appiled po- Percent e in insula g .due to tion showsuperiority over one given a simple Conductor treated :gli appliedpoten- Conductor` treated without applied potential..

Percentage increase duc to application of potential It will be notedthat the insulating conductor prepared in accordance with theinvenimpregnation with cellulose acetate particularly at the hi lierhumidities and also shows a greater ten ency to recover rapidl `from theeffects of humidity. The varying umidity conditions to which theconductors were subjected throuohout the test While severe, neverthelessindicate the conditions to which the switchboard cable maybe subjectedin actual operating conditions.

While the invention has been described with particular reference to,cotton covered conductors, it isy to be understood that it is equallyapplicable to conductors insulated with other materials.

The invention is also capable of other modifications and adaptations notspecifically re-4 ferred to but included within the scope of theappended claim.

What is claimed is: s

The method of treating an insulated conductor which com rises immersingthe conductor in a bath o? cellulose acetate, causing a ilow of electriccurrent through the conductor and the bath, and subsequently insuccession passing the conductor through a bath of cellulose acetate andthrough a drying chamber.

In witness whereof, I hereunto subscribe my name this Qndday of NovemberA. D.,

DAVID R. BROBST.

